KR20160048134A - Co slip catalyst and method of using - Google Patents

Abstract

A CO slip catalyst for treating exhaust gas from a lean burn internal combustion engine is disclosed. The CO slip catalyst comprises palladium and ceria containing materials. The present invention also includes a method of oxidizing excess CO in the exhaust gas, wherein excess CO results from cyclic contact of the upstream catalyst under rich exhaust conditions. The method comprises contacting excess CO2 in the exhaust gas with a CO slip catalyst at a temperature in the range of 100 to 700 < 0 > C.

Description

CO SLIP CATALYST AND METHOD OF USING [0002]

The present invention relates to a CO slip catalyst for treating exhaust gas from a lean burn internal combustion engine and a method for treating excess CO in the exhaust gas.

The internal combustion engine produces exhaust gases containing various pollutants including hydrocarbons, carbon monoxide (CO), nitrogen oxides, sulfur oxides, and particulate matter. Increasingly stringent national and local laws have reduced the amount of pollutants that can be released from these internal combustion engines. Many other techniques have been applied to exhaust systems to clean the exhaust gases before they are released to the atmosphere.

One proposed method for purifying exhaust gases produced in lean-burn applications uses a three-way catalyst (TWC) followed by a selective catalytic reduction (SCR) catalyst. This concept, such as that described in U.S. Patent No. 8,216,521, is for lean operation during an extended period of time in which CO and hydrocarbons in the engine exhaust stream are primarily oxidized to CO ₂ and H ₂ O on the TWC. A periodic rich situation is used to produce ammonia (NH ₃ ) on the TWC. NH ₃ is stored and used by downstream SCR catalysts to selectively reduce NO _x generated during the lean step. The duration of these enriched conditions requires optimization to provide the appropriate amount of NH ₃ to the SCR to meet the NO _x conversion target. Unfortunately, the effect of longer, thicker conditions is the generation of excess CO in the exhaust. It is important to maintain high conversions of hydrocarbons, NO _x , and CO during both the lean and rich operating stages. As the rich pulse period becomes longer to optimize NH ₃ generation, CO is the most difficult to control using only TWC.

It is required to develop new and improved catalysts and methods that enable longer thicker conditions while maintaining a high level of CO conversion. The present inventors have developed a new CO slip catalyst capable of converting a high concentration of CO generated during rich operation.

The present invention is a CO slip catalyst for treating exhaust gas from a lean burn internal combustion engine. The CO slip catalyst comprises palladium and ceria containing materials. The present invention also includes a method of oxidizing excess CO in the exhaust gas, wherein excess CO results from cyclic contact of the upstream catalyst under rich exhaust conditions. The method comprises contacting excess CO2 in the exhaust gas with a CO slip catalyst at a temperature in the range of 100 to 700 < 0 > C. The present invention leads to an improved conversion of CO during the lean and rich operating phases.

The present invention is a CO slip catalyst. The CO slip catalyst comprises palladium and ceria containing materials. Preferably, the CO slip catalyst consists essentially of, and more preferably, a palladium and ceria-containing material. The ceria-containing material is preferably ceria, ceria-zirconia, ceria-zirconia-alumina, or mixtures thereof. More preferably, the ceria-containing material is ceria.

The CO slip catalyst preferably comprises 0.1 to 30 weight percent palladium, more preferably 0.5 to 5 weight percent palladium, and most preferably 1 to 4 weight percent palladium. The CO slip catalyst may contain other noble metals such as platinum and rhodium, and palladium is preferred as the only noble metal.

Preferably, the CO slip catalyst may further comprise an accelerator such as an alkali metal and an alkaline earth metal. Preferred alkaline earth metals include barium, calcium, strontium, or magnesium. Preferred alkali metals include potassium, sodium, lithium, or cesium. Other promoters such as copper and zinc may be added instead of or in addition to the alkali metal or alkaline earth metal promoter.

The CO slip catalyst may preferably comprise one or more inorganic oxide binders. Preferred inorganic oxide binders are alumina, silica, titania, zirconia, magnesia, niobia, tantalum oxide, molybdenum oxide, tungsten oxide, any two or more mixed oxides or complex oxides of these (for example silica-alumina or magnesia- And mixtures thereof.

The CO slip catalyst is preferably coated over the substrate. The substrate is preferably a ceramic substrate or a metal substrate. The ceramic substrate can be made of any suitable refractory material such as alumina, silica, titania, ceria, zirconia, magnesia, zeolite, silicon nitride, silicon carbide, zirconium silicate, magnesium silicate, aluminosilicate, (Such as cordierite and spodumene), or mixtures of two or more of these, or mixed oxides. Cordierite, magnesium aluminosilicate, and silicon carbide are particularly preferred.

The metal substrate may be made of any suitable metal and may be made of a heat resistant metal and metal alloy, such as titanium and stainless steel, as well as a ferrite alloy containing iron, nickel, chromium, and / Can be made.

The substrate may be a filter substrate or a flow-through substrate, and most preferably a penetrating substrate, in particular a honeycomb monolith. The substrate is typically designed to provide numerous channels through which the vehicle exhaust passes. The surfaces of the channels are loaded with catalyst.

The CO slip catalyst of the present invention can be prepared by processes well known in the art. CO slip catalysts are preferably prepared to ensure a high level of interaction between the palladium particles and the ceria-containing material particles. This can be achieved by impregnation with a ceria-containing material phase using a palladium compound (such as palladium nitrate) followed by calcination. Alternatively, this high interaction may be produced by coprecipitation of palladium and ceria-containing material particles.

Preferably, the CO slip catalyst is prepared by depositing it on a substrate using a washcoat process. A representative process for producing a CO slip catalyst using a washcoat process is presented below. It will be appreciated that the following process may be varied according to other embodiments of the present invention.

The CO slip catalyst is preferably produced using a washcoat process. Palladium compounds (such as palladium acetate or palladium nitrate) are preferably added thereto prior to the washcoating step, where ceria-containing materials and binders are used. The palladium compound can be loaded onto the ceria-containing material by any known means, and the mode of addition is not considered particularly important. For example, a palladium compound may be added to the ceria-containing material by impregnation, adsorption, ion exchange, initial wetting, precipitation, etc. to produce supported palladium materials. Alternatively, the ceria containing material and the binder, if used, can be coated on the substrate, and then the palladium compound can be added to the coated substrate.

Wash coating is preferably carried out by first slurrying the finely divided particles of the supported palladium material (or only ceria containing material) and the optional binder to form a slurry in an appropriate solvent, preferably water. The slurry preferably contains 5 to 70 weight percent solids, more preferably 10 to 50 weight percent. Preferably, prior to forming the slurry, the particles are milled or otherwise subjected to a grinding process to ensure that substantially all of the solid particles have less than 20 microns in average diameter. Additional components, such as accelerators, may also be included in the slurry as a water soluble or water dispersible compound or a mixture of complexes.

The substrate can then be coated one or more times with the slurry to deposit the desired loading of the catalyst material on the catalyst onto the substrate. If the ceria-containing material and optional binder are deposited on the substrate, the palladium compound is then added to the coated substrate by any known means, including impregnation, adsorption, or ion exchange of a platinum compound (such as platinum nitrate) .

Preferably, the entire length of the substrate is coated with a slurry so that the washcoat of the CO slip catalyst covers the entire surface of the substrate.

After the catalyst is deposited on the substrate, the CO slip catalyst is typically calcined by drying and then heating at a high temperature. Preferably, calcination occurs at 300 to 700 < 0 > C for approximately 1 to 8 hours.

The CO slip catalyst may be a single layer catalyst preparation or may be prepared as a multi-layer catalyst. For example, the CO slip catalyst may be applied as two or more layers on a substrate with each layer having a different amount of palladium and / or ceria-containing material. The CO slip catalyst may also be added to another catalyst component either as a lower layer or as an upper layer or as a rear zone of another catalyst component, for example as a lower or rear zone on a selective catalytic reduction catalyst.

The present invention also includes a method of oxidizing excess CO in the exhaust gas, wherein the excess CO results from periodic contact of the upstream catalyst under rich exhaust conditions. The process of the present invention comprises contacting excess CO with a CO slip catalyst at a temperature in the range of from 100 to 700 < 0 > C.

A typical method of defining the composition balance of the exhaust gas containing both the oxidizing gas and the reducing gas is the lambda (?) Value of the exhaust gas. The lambda value is defined as the actual engine air-to-fuel ratio / stoichiometric engine air-to-fuel ratio, where a lambda value of 1 represents a stoichiometrically balanced (or stoichiometric) exhaust gas composition. A lambda value (lambda> 1) greater than 1 represents excess O ₂ and NO _x and the composition is described as "lean". A lambda value of less than 1 (lambda <1) represents excess hydrocarbon and CO and the composition is described as "rich".

In the Linburn internal combustion engine, in order to regenerate the exhaust catalyst components, it is necessary to remove the NO _x species adsorbed to the catalyst component and to generate ammonia for use in other downstream catalyst components, ). &Lt; / RTI &gt; For example, during extended lean operation, CO and hydrocarbons in the engine exhaust gas stream will be oxidized primarily to CO ₂ and H ₂ O on a three-way catalyst (TWC) component. Periodic rich conditions can be used to generate ammonia (NH ₃ ) on the TWC. NH ₃ is stored and used by downstream selective catalytic reduction (SCR) catalysts to selectively reduce the NO _x generated during the lean step. These abundance situations require optimization to provide an appropriate amount of NH ₃ to the SCR to meet the NO _x conversion target. As a consequence of the longer rich event, excess CO evolution occurs in the exhaust. The CO slip catalyst of the present invention helps to oxidize this excess CO to maintain a higher conversion of CO during both the lean operation and the rich operating phase.

In the process of the present invention, excess CO results from periodic contact of the upstream catalyst under rich exhaust conditions. Preferably, the upstream catalyst comprises a three-way catalyst (TWC).

Three-way catalyst systems are well known in the art. TWC typically has three main functions: (1) oxidation of CO to CO ₂ ; (2) oxidation of unburned fuel to CO ₂ and H ₂ O; And (3) reduction of NO _x to N ₂ . The ternary catalyst preferably comprises at least one platinum group metal and at least one inorganic oxide support. The platinum group metal (PGM) is preferably platinum, palladium, rhodium, or mixtures thereof.

The inorganic oxide support most typically comprises oxides of elements 2, 3, 4, 5, 6, 13 and 14 and lanthanide elements. Useful inorganic oxide supports preferably have a surface area in the range of 10 to 700 m ² / g, a pore volume in the range of 0.1 to 4 mL / g, and a pore diameter of about 10 to 1000 angstroms. The inorganic oxide supports are preferably alumina, silica, titania, zirconia, ceria, niobia, tantalum oxide, molybdenum oxide, tungsten oxide or any two or more mixed oxides or complex oxides thereof such as silica- Zirconia or alumina-ceria-zirconia. Alumina and ceria are particularly preferred. In addition to serving as a support, ceria containing supports such as ceria (CeO ₂ ) or ceria-zirconia mixed oxides may also function as oxygen storage component (OSC) within the TWC. The inorganic oxide support may also be a zeolite such as beta zeolite, ZSM zeolite, ferrierite, or carbazite.

The ternary catalyst is preferably coated on the substrate as described above. The substrate can be a filter substrate or a penetrating substrate, and most preferably a penetrating substrate, especially a honeycomb monolith. The substrate is typically designed to provide numerous channels through which the vehicle exhaust passes. The surfaces of the channels are loaded with a three-way catalyst.

The ternary catalyst may be added to the substrate by any known means. For example, an inorganic oxide support or a PGM-containing support material can be applied and bonded to a substrate as a washcoat, wherein a porous, high surface area layer is bonded to the surface of the substrate. Washcoats are typically applied to substrates from aqueous slurries, then dried and calcined at elevated temperatures. If the inorganic oxide support is washcoated on the substrate, the PGM metal can be loaded (impregnated, ion exchanged, etc.) onto the dried washcoat support layer, then dried and calcined. The preferred loading of the PGM loading in the substrate is 0.02 to 1.7 g / liter (1 to 300 g / ft ³⁾ a catalyst volume.

The upstream catalyst preferably further comprises a selective catalytic reduction (SCR) catalyst located downstream of the TWC. SCR catalysts are catalysts that reduce NO _x to N ₂ by reaction with nitrogen compounds (such as ammonia or urea) or hydrocarbons (lean NO _x reduction). Preferably, the SCR catalyst comprises a vanadia-titania catalyst, a vanadia-tungsten-titania catalyst, or a transition metal / molecular sieve catalyst. Transition metal / molecular sieve catalysts include transition metals and molecular sieves, such as aluminosilicate zeolites and silicoaluminophosphates.

Preferred transition metals include chromium, cerium, manganese, iron, cobalt, nickel and copper, and mixtures of any two or more thereof. Iron and copper are particularly preferred. Zeolite, ZSM zeolite (e.g., ZSM-5, ZSM-48 (for example, zeolite) ), SSZ-zeolites (e.g. SSZ-13, SSZ-41, SSZ-33), ferrierite, mordenite, carbazite, opretite, erionite, clinoptilolite, (Including metal aluminophosphates such as SAPO-34), mesoporous zeolites (e.g., MCM-41, MCM-49, SBA-15), or mixtures thereof; More preferably, the molecular sieve is beta zeolite, ferrierite, or carbazite. Preferred SCR catalysts include Cu-CHA, such as Cu-SAPO-34, Cu-SSZ-13, and Fe-beta zeolite.

The SCR catalyst is preferably coated onto a ceramic or metal substrate. The substrate is typically designed to provide numerous channels through which vehicle exhaust passes, and the surfaces of the channels will preferably be coated with an SCR catalyst.

The substrate for the SCR catalyst may be a filter substrate or a penetrating substrate. Preferably, the SCR catalyst is coated on the filter. The combination of SCR catalyst and filter is known as a selective catalytic reduction filter. The selective catalytic reduction filter is a single substrate device that combines the functionality of the SCR and the particulate filter.

The following examples merely illustrate the invention. Those skilled in the art will recognize many variations within the scope of the inventive concepts and claims.

Example  1: Pd / Celia  Preparation of CO slip catalyst

Catalyst 1 is prepared by coating a CO slip catalyst having major components CeO ₂ , Al ₂ O ₃ , and Pd (3 wt.%) On a cordierite honeycomb monolith.

Example  2: Laboratory test procedures and results

Catalyst 1 and comparative standard Pd-Rh TWC are tested for CO oxidation activity. The reactor core from the CO slip catalyst was evaluated by hot aging at 750 ° C for 16 hours and compared to the performance of the new comparative standard TWC reactor core. The experiment was carried out using the following procedure.

A lean (0% CO, 10% O 2, 5% H 2 O, 8% CO 2) and enriched (1.5% CO, 0% O 2, 5% H 2 O, 8% CO 2) condition during the catalyst test A gas mixture for simulation is prepared. The gas flow is controlled to achieve a space velocity of 30,000 hr &lt; ^-1 &gt;. During the test, each sample is pre-conditioned in a lean gaseous mixture, at which the catalyst inlet temperature is raised to 500 ° C and held for 5 minutes. Following this pretreatment step, the catalyst is stabilized at 100 ° C under lean conditions, where performance is measured during 10 cycles (5 minutes in lean conditions, 30 seconds in rich conditions). This cycle performance is also measured at 150, 200, 250, and 300 ° C. The results are shown in Table 1.

It can be seen that the effect of hydrothermal aging on CO conversion is very small at almost all the temperatures studied. This catalyst is also significantly better than the standard TWC under conditions of both fresh / aged.

Time for 1000 ppm CO slip at various temperatures
catalyst
1000 ppm  Time for CO sleep (seconds)
100 ℃ 150 ℃ 200 ℃ 250 ℃ 300 ° C New TWC * 2 3 5.5 7 10.5 Catalyst 1 - New 14.5 18.25 20.5 24.75 26.25 Catalyst 1 - aged 17.5 19.75 21 24.25 24.75

* Comparative Example

Claims (15)

A CO slip catalyst for treating exhaust gas from a lean burn internal combustion engine, the CO slip catalyst comprising palladium and a ceria containing material. The CO slip catalyst according to claim 1, wherein the ceria-containing material comprises ceria, ceria-zirconia, ceria-zirconia-alumina, and mixtures thereof. The CO slip catalyst according to claim 2, wherein the ceria-containing material comprises ceria. The CO slip catalyst according to claim 1, wherein the CO slip catalyst comprises 0.5 to 5 weight percent palladium. The CO slip catalyst according to claim 1, further comprising at least one alkali metal or alkaline earth metal. The method according to claim 1, further comprising at least one inorganic oxide binder selected from the group consisting of alumina, silica, titania, zirconia, magnesia, niobia, tantalum oxide, molybdenum oxide, tungsten oxide and any two or more mixed oxides thereof Wherein the CO slip catalyst is a CO slip catalyst. The CO slip catalyst according to claim 1, wherein the CO slip catalyst is coated on the substrate. The CO slip catalyst according to claim 7, wherein the substrate is a ceramic substrate or a metal substrate. A method for oxidizing excess CO in exhaust gas from a lean burn internal combustion engine wherein the excess CO results from cyclic contact of the upstream catalyst under rich exhaust conditions and wherein the process is carried out at a temperature in the range of from 100 to 700 & Contacting the excess CO with a CO slip catalyst, wherein the CO slip catalyst comprises palladium and ceria containing material. 10. The method of claim 9, wherein the ceria-containing material comprises ceria, ceria-zirconia, ceria-zirconia-alumina, and mixtures thereof. 11. The method of claim 10, wherein the ceria containing material comprises ceria. 10. The process of claim 9, wherein the CO slip catalyst comprises 0.5 to 5 weight percent palladium. 10. The method of claim 9, further comprising at least one alkali metal or alkaline earth metal. The method according to claim 9, further comprising at least one inorganic oxide binder selected from the group consisting of alumina, silica, titania, zirconia, magnesia, niobia, tantalum oxide, molybdenum oxide, tungsten oxide and any two or more mixed oxides thereof . &Lt; / RTI &gt; 10. The method of claim 9, wherein the CO slip catalyst is coated on the substrate.